The present invention pertains to fasteners and more particularly to frangible fasteners for use in an air bag type safety device deployment system.
Vehicular safety is extremely important and therefore provides motivation for continued development in safety devices to improve vehicular safety. Many types of occupant safety devices have been developed in order to protect occupants of vehicles from injuries as a result of primary and secondary collisions. For example, bodily restraints such as seat belts have been employed in cars for many years in order to prevent the occupants from being injured from a secondary collision. The secondary collision results from the occupant being thrown into a surface inside of the car, such as a steering wheel, dashboard, or windshield. Secondary collision injuries can be extremely debilitating, disfiguring, or even fatal since many of these injuries involve damage to the head or face.
Air bag safety device technology has developed in order to protect vehicle occupants from the effects of secondary collisions. An air bag type safety device involves a large inflatable bag, some form of inflating device, and a storage compartment. "Driver side" air bags have been developed that are contained within the hub of the steering wheel. This type of air bag is extremely effective such that when the air bag is deployed and inflated, it is positioned immediately in front of the driver and between the driver and the steering wheel. The deployed air bag cushions the driver's inevitable secondary impact with the steering wheel.
Several types of air bag deployment systems are available. One type of system uses a controllable combustible fuel cell which is ignited in response to an impact detected by a sensor. Once the fuel cell is ignited, non-combustible gases created by the combustion process flow through a filtering device to clean and cool the gases and inflate the air bag. The air bag is deployed and inflated in approximately 1/20th of a second and then deflated after the collision. Typically, the air bag is inflated with some form of nitrogen or other non-combustible gas.
In the driver side air bag arrangement, a cover panel covers a compartment located in the hub of the steering wheel wherein the air bag is stored. During the deployment of the air bag, the cover panel is forced off of the storage compartment, yet remains attached to the hub, as a result of forces created by the inflating air bag. The reason for retaining the panel on the hub is to prevent a loose component from becoming a potentially harmful projectile in the passenger compartment during a collision.
A similar arrangement is provided for a passenger side air bag. On the passenger side, a compartment or an opening is provided through the dashboard in front of the passenger position. The opening is covered with a releasable cover panel which is attached to the dashboard by means of some form of tether and hold down fasteners. The primary concern with regard to the passenger side air bag is to prevent the passenger from impacting the dashboard as well as the windshield.
It would be desirable to provide a fastener which will retain the cover panel over the air bag compartment until deployment of the air bag. Upon deployment of an air bag from the compartment, it would be desirable to provide a fastener which is frangible or fracturable to permit the air bag to be deployed. It would also be desirable to provide a fastener which prevents tampering with the air bag device such that the fastener cannot be loosened or removed with commonly available tools. As an additional matter, it is extremely important to provide a fastener which fractures but does not create loose projectiles during the deployment operation. As mentioned above, projectiles, and especially small projectiles, could potentially injure the passengers of the vehicle.
One form of prior art fastener device has been employed for retaining a cover panel over an air bag compartment. These types of fasteners are formed with a shank which have annular rings formed generally in a perpendicular plane and radially extend from the outside surface of the shank. The annular rings are typically designed to push back or flex upon inserting the fastener into the appropriate holes in the air bag compartment and cover panel. Such fasteners have been referred to as "Christmas tree" fasteners because when the fastener is cross-sectioned longitudinally through the central axis of the fastener, a Christmas tree type silhouette is produced.
While this type of fastener has been used to retain a cover, a problem arises such that the fastener may not be sufficiently strong to resist separation tension forces which could result in a portion of the fastener becoming a free projectile in the passenger compartment. Another problem with this type of fastener is that the fastener tends to ratchet through a hole in the member to which it is attached. This ratcheting effect resulting in a loose fit in the hole which, when combined with ordinary vehicular vibrations, produces a rattling sound. Additionally, a problem arises with these types of fasteners because the fasteners tend to fracture irregularly.